1. Field of the Invention
The present invention relates to an optical monitor for use in a reception unit of an optical line terminal (OLT) in a high speed passive optical network (PON).
2. Description of the Related Art
FIG. 1 is a diagram describing a PON telecommunication system.
The PON telecommunication system is one in which an optical star coupler 10 receives an optical signal from each of clients ONU #1 through #3 and transmits it to an OLT 11. An optical signal from each of the clients ONU#1 through #3 is in a packet form. Only the optical star coupler 10 is installed between the clients ONU#1 through #3 and the OLT 11, and an active apparatus such as an optical amplifier is not installed. The individual clients ONU#1 through #3 send out optical packet signals at timings determined between themselves and the OLT 11.
Here, an optical packet signal sent from each of the clients ONU#1 through #3 is input to the optical star coupler 10 with a different loss caused by each transmission path because the characteristic and/or length of an optical transmission path connecting between each of the clients ONU#1 through #3 and the optical star coupler 10 are different. That is, an optical packet signal propagating through a transmission path with a low loss remains as a high optical power when it is mixed with other optical packet signals at the optical star coupler 10, while one propagating through a transmission path with a high loss remains as a low optical power when it is mixed with other optical packet signals at the optical star coupler 10. Therefore, optical signals sent from the optical star coupler 10 to the OLT 10 are burst signals with different optical powers for each of the packet signals.
In such a burst telecommunication, since optical powers of optical packet signals from individual client ONU #1 through #3 are different, it is possible to obtain information of the path from each client in real time if an optical power of each optical packet signal can be monitored for each packet. Therefore, it is desired to measure an optical power for each packet in the above described PON telecommunication system.
A conventional power monitor is mainly the function of monitoring an average optical power of an optical signal over a predetermined time period and outputting it, whereas a function of monitoring an optical power for each packet is a technique that is not conventionally available.
A patent document 1 has noted a technique for receiving a preamble bit preceding a burst signal and controlling an amplifier for a PON system.
[Patent document 1] Japanese Patent Application Publication No. 2000-151290
Since an optical signal intensity-modulated causes a difference of optical power between a “0” sign and a “1” sign, an input optical power is specified by an intermediate optical power between the “0” and “1”. However, if long bits of the same sign are included in a data part of an optical packet, the average optical power is biased toward an optical power of the “0” or “1”, hence making impossible to evaluate an intermediate value during such a period.